Optical Fiber Coatings
4.3 EVOLUTION OF OPTICAL FIBERS AND PROTECTIVE COATINGS
Attenuation losses associated with optical fibers continued to decline during the 1970s: 4 dB/km (1975), 0.5 dB/km (1976), and 0.2 dB/km (1979). The latter value corresponds to 63% of a light signal reaching the end of a 10-km long fiber [20]. In 1982, Corning achieved an attenuation of 0.16 dB/km, on single-mode fibers transmitting at 1550 nm, representing a 100-fold improvement over the 1970 transmission of Mauer’s first low-loss fiber [21]. It has been reported that if the ocean had the same transparency as the glass in such low-loss fibers, one could see to the bottom of the Mariana Trench in the Pacific Ocean, a little more than 6 miles below sea level [22].
4.3.1 Coating Contributions to Microbending Minimization
Concurrent with the aforementioned improvement in fiber transmission prop-erties, fiber coatings evolved from single-layer to dual-layer systems. In the early 1980s, the outer diameter for dual-layer systems was standardized to between
245 and 250 microns, while the outer diameter for the inner coating, contacting the glass, ranged from 190 to 210 microns. The dual-layer coating system was designed to enhance protection for fibers against microbending-induced attenu-ation. This phenomenon is caused by microscopic departures from straightness in the waveguide axis [23]. Varying causes of microbending include longitudinal shrinkage of the fiber coating, poor drawing or cable manufacturing methods, or stresses imposed during cable installation [24].
D. Gloge [25] first reported that microbending, losses could be reduced by shielding the fiber from outside forces by using a soft inner coating, having a modulus of 14,000 psi (100 MPa), and an outer shell of a material having a modulus of 140,000 psi (1000 MPa). The inner primary coating is designed to act as a shock absorber, under the tougher outer layer, to minimize attenuation caused by microbending. It has a very low crosslink density and current primary coatings typically have a modulus between 0.5 and 3.0 MPa. It must adhere to the glass, yet strip cleanly from the glass, to facilitate splicing and connecting.
The outer primary coating, sometimes called the secondary coating, protects the primary coating against mechanical damage and acts as a barrier to lateral forces. It also serves as a barrier to moisture. It is a hard coating, having a high modulus and Tg, to facilitate good handling and durability. It is generally fast curing, for ease of processing, and has good chemical resistance to solvents, cable filling gels, and moisture. The surface properties of the secondary coating must be carefully controlled to allow good adhesion of the ink used in color identifi-cation while allowing for good winding onto takeup spools. A schematic dia-gram of an optical fiber is shown in Fig. 4.1.
During the development of dual-layer coating systems, it was important to consider the modulus of the inner coating not only at room temperature but also at colder temperatures to which fibers could reasonably be exposed [12]. Visco-elastic coating materials are known to increase in modulus as temperature drops (i.e., they become stiffer). If these coatings also adhere tightly to glass, they can impose forces on the fiber that will produce microbending-induced signal attenuation.
Primary Coating OD = 190 mm
Cladding OD = 125 mm Glass Core
OD = 8-10 mm (single mode)
= 50 or 62.5 mm (multimode)
Secondary Coating OD = 250 mm
Color Coating Thickness = 3-5 mm Figure 4.1 Schematic of coated fiber cross-section.
Kuzushita et al. [26] reported on the low-temperature modulus properties of nine coatings and correlated them with the added attenuation observed at
30C for fibers coated with four resin types: polyester-polyol–type urethane acrylate, polyether-polyol–type urethane acrylate, polybutadiene acrylate, and UV-curable silicone.
The modulus of coating films at cold temperatures was initially determined by Instron tensile strength testing of films in temperature-controlled chambers. This process proved fairly time consuming, because of the time required for tempera-ture equilibration between runs.
The development of dynamic mechanical analyzers gave rise to the exceed-ingly more efficient dynamic mechanical analysis (DMA) of UV-cured films.
This nondestructive test allows for temperature sweeps at a chosen frequency to define a material’s modulus as a function of temperature. The slope of a coat-ing’s modulus curve as it changes from the glassy phase to the rubbery phase can, in part, determine the suitability of coatings for use at low temperatures. Sarkar et al. [27] shared examples of DMA curves for soft RTV silicone and UV-cured urethane acrylate primary coatings and showed how they compared with the attenuation properties of fibers on which they were applied. This reference also illustrates improved temperature-induced attenuation at 1300 nm for the UV-cured urethane acrylate coating, which exhibited a significantly lower modulus profile at lower temperatures.
4.3.2 Glass Fiber Fracture Mechanics and Coating Contributions to Fiber Strength Retention
Fiber fatigue is an important mechanical property of optical fibers. Pristine silica fibers have strengths of approximately 7 GPa at ambient condition. How-ever, fibers can experience fatigue when subjected to lower stresses for long periods. Fiber fatigue is thought to occur by crack growth of existing flaws on the glass surface due to interaction between the Si-O bond and the moisture in the environment when the fiber is subjected to stress.
The coating is thought to contribute to fiber fatigue in that basic compounds present in the composition can accelerate glass corrosion. Conversely, acidic components have been shown to improve fatigue resistance. Skutnik et al. [28]
found that coatings with strong adhesion facilitated a greater n-value fatigue parameter for coated fibers. Supporting this is the observation that both n-value and adhesion generally increase with time after draw.
When measuring fatigue, it is typical to measure either static fatigue or dynamic fatigue. In static fatigue, a constant stress is applied to the fiber, and the time to failure is measured. In dynamic fatigue, the strength of the fiber is measured as a function of the applied stress rate.
There are two techniques for measuring dynamic fatigue: tension and two-point bending. In the tensile test, a fiber is gripped at each end and pulled in tension until it breaks. In the two-point bending test, fibers are bent between two faceplates that move toward each other at a controlled rate until the fibers break [29–32].
Gulati [33] published one of the earliest papers that discussed test methods for measuring the tensile and bending strengths of optical fibers. He also calculated the required proof stress level to ensure fiber durability of at least 20 years when subjected to a known value of service stress.
Michalske and Bunker [34–38] published a number of studies on the fracture mechanics of glass and glass fibers. Helfinstine [39] published a very thorough review on delayed failure or sub-critical crack growth in glass.
Wang and Zupko [17] found fiber strength retention to be a function of fiber wetting and adhesion by protective polymeric coatings. Fiber strength decreased when application viscosity was increased and wetting of the fiber was conse-quently decreased.
Sakaguchi et al. [40] demonstrated that silane treatment stabilizes glass surfaces against water. The group evaluated the behavior of untreated, silicone-treated, and silane-treated glass in water at different temperatures.
Infrared spectroscopy was used to monitor the interaction of water molecules with glass silanol groups.
Schlef et al. [41] evaluated the performance of a number of UV-curable primary and secondary coatings via static fatigue, dynamic tensile testing, and proof testing and concluded that these coatings ‘‘retain the initial strength and fatigue resistance of optical fibers.’’
Dunn and Smith [42] performed a variety of abrasion and static fatigue tests, demonstrating that the use of hard UV-cured secondary coatings yielded fibers with improved strength and handling characteristics, compared with silicone single coatings.
4.3.3 Durability of Fiber Optic Coatings
Long-term durability of protective coatings was considered to be of consider-able importance because fibers installed into outside plant networks were expected to have a minimum service lifetime of 25 years [43–45]. D. R. Young [46] provided ‘‘the first report on a long-term, long length static fatigue test in an outdoor, in-ground trough environment.’’ He reported that UV-cured acrylate composite protective coatings were shown to provide excellent protection against a variety of environments: 65C air, high and low pH solutions, and temperature/
humidity cycling of 150 days 65C=98% relative humidity (RH) to 10C=4% RH.
Weibull plots, which describe the probability of failure at a given stress for a given length, were provided for fibers exposed to temperature/humidity cycling
and immersed in petroleum jelly, similar to what is used as loose tube filling compound.
In 1984, O. R. Cutler [47] reported on the results of his durability testing on UV-curable coating films. In his study, Cutler [47] exposed 75 mm thick films of a variety of commercial fiber coatings to temperatures of 38, 54, 88, 125, and 175C, for up to 1 year. He developed Arrhenius plots that showed, based on the time required to double the coating’s modulus, operating lifetimes of primary and secondary coatings could extend beyond 100 years at room temperature.
Several coatings had similar calculated service lifetime when aged continuously at 54C (130F).
Nevins and Taylor [43] issued their report on the effect of a variety of environ-mental conditions on the ‘‘three key characteristics of fiber optic waveguides which may be effected by environmental conditions: strength, attenuation and resistance to losses caused by microbending.’’ In addition to the conditions reported earlier by Young [46], the research team of Nevin and Taylor mentioned exposure to seawater, fungi, and abrasives. They also cited an experiment, Pro-custes, designed to correlate accelerated aging tests with actual long-term aging.
Simoff et al. [48] studied the aging of a polyether urethane acrylate primary coating, both in films and on fiber, and correlated the changes in physical properties of the films with the stripping force required to remove the coating from the fiber.
In 1993, Chawla et al. [49] reported on fiber optic coating durability, as was measured by weight changes and shifts in DMA modulus profiles. DMA pro-vides insight into a material’s durability following exposure to a wide variety of environments such as hydrolytic, thermo-oxidative, and chemical exposure.
Comparison of DMAs before and after exposure to these conditions allows one to monitor changes in the material’s glass transition temperature profile and the material’s equilibrium modulus. The equilibrium modulus region of the DMA curve is observed at the modulus plateau reached in the rubbery phase.
This modulus can be related to the crosslink density of a cured coating’s network through the equation
r0¼ E0=6kT , (4:1)
where r0represents crosslink density, k is the Boltzmann constant, and T is the temperature, in degrees-Kelvin.
Decreases in equilibrium modulus indicate a reduction of a coating network’s crosslink density through chain scission, and hence a weaker coating. Con-versely, increases in the equilibrium modulus can signify embrittlement of the coating network through crosslinking. Chawla et al. [49] demonstrated excellent durability of several primary coatings after 1 year of aging at 125C.
The results of additional durability studies [44, 45, 50–56] have been published and offer a deeper understanding of this subject.